The main methods for measuring the electrical parameters of piezoelectric resonators are sometimes classified in two categories, "active" and "passive".
Active methods are based on oscillator circuits in which the resonator is the frequency determining element, and where a voltage and current at the resonance frequency build up across and through the resonator, starting at near-zero amplitude and building up during a transition phase to a steady-state value by means of amplification in a feedback loop.
Passive methods are based on applying a constant-amplitude voltage of predetermined frequency across the resonator, causing the resonator current to rise from a near-zero value during a transition time to a steady-state value.
Active methods are fast but inaccurate. For accurate parameter measurements, passive methods are commonly used. Of these there are several types, such as transmission, reflection, and S-parameter methods. They employ a variable-frequency RF voltage generator and a vector voltmeter to measure the resonator impedance at several frequencies by applying a constant-amplitude voltage at a frequency at or near resonance and making at least one more similar measurement at another near-by frequency and another measurement at a frequency far removed from resonance. The resonator parameters can then be calculated from these measurements. The methods have been internationally standardized, as described in "Publication 444", published by the International Electrotechnical Commission.
FIG. 1 is adapted from IEC Publication 444-5 page 59 figure A4. It shows a transmission measurement circuit. A network analyzer 1 includes an RF voltage generator with an output at its terminal C and a vector voltmeter with two inputs at terminals A and B. Terminal C is connected to a power splitter 3, which divides the power into two paths. One path is connected to a resonator 2 via a "Pi Network", a resistive network used to match the resonator impedance to that of its external connections. The output terminal of the pi network is connected to the B input of the vector voltmeter. The other path is connected to input A of the vector voltmeter via an attenuator 5. The vector voltmeter measures the amplitude of each input and the phase of the B input relative to the A input.
FIG. 2 shows a simplified circuit that is similar to that of FIG. 1. As in FIG. 1, an RF generator 4 is again connected to two paths: one to a resonator 2 and the other to input A of a vector voltmeter 7. The pi network is replaced by a resistor 8 and the internal impedance of generator 4. Current through the resonator produces a voltage across resistor 8 that is proportional to and representative of the current. That voltage is increased by amplifier 10 and applied to input B of the vector voltmeter. The circuit of FIG. 2 will be used to show how the invention can be incorporated into a transmission measurement system. A parameter evaluation can take several forms, but it requires at least three measurements. A common approach is to make measurements at two different frequencies at or near resonance, and a third measurement at an off-resonance frequency. All prior-art systems for parameter measurements apply a constant, steady-state voltage to the resonator.
The disadvantage of passive measurements is that they take longer than oscillator measurements. When the constant voltage is applied to the resonator, the resonator current builds up gradually until it reaches a constant steady-state value at which the measurement can be taken. The delay time depends on the resonator parameters and increases with decreasing resonator frequency. For example, for a crystal with a quality factor Q=100,000, a resistance of 10 Kohm, and a frequency of 32 kHz, the delay time is approximately 7 seconds. Since the parameter evaluation requires at least three measurements at different frequencies, the conventional method is quite time consuming.
The present application describes means for significantly reducing the measurement delay.